U.S. patent application number 12/599287 was filed with the patent office on 2012-04-26 for transducer.
This patent application is currently assigned to CAMBRIDGE INTEGRATED CIRCUITS LIMITED. Invention is credited to David Thomas Eliot Ely.
Application Number | 20120098527 12/599287 |
Document ID | / |
Family ID | 40002688 |
Filed Date | 2012-04-26 |
United States Patent
Application |
20120098527 |
Kind Code |
A1 |
Ely; David Thomas Eliot |
April 26, 2012 |
TRANSDUCER
Abstract
A transducer is described for use in a position sensor. The
transducer has a magnetic field concentrating member, such as an
elongate ferrite rod, a first coil operable, in use, to couple with
a first portion of the magnetic field concentrating member
positioned adjacent the first coil and a second coil operable, in
use, to couple with a second portion of the magnetic field
concentrating member positioned adjacent the second coil, the
second portion being spaced along the magnetic field concentrating
member from the first portion. The transducer also comprises a
resonator having a coil wound around a portion of said magnetic
field concentrating member which is located between said first and
second portions. The transducer is arranged so that, during use,
the electromagnetic coupling between the resonator and at least one
of the first and second coils varies as a function of the relative
position between the elongate field concentrating member and that
coil. In this way, separate processing electronics can process the
signals obtained from the transducer to determine the desired
position information.
Inventors: |
Ely; David Thomas Eliot;
(Cambridge, GB) |
Assignee: |
CAMBRIDGE INTEGRATED CIRCUITS
LIMITED
Cambridge, Cambridgeshire
GB
|
Family ID: |
40002688 |
Appl. No.: |
12/599287 |
Filed: |
April 28, 2008 |
PCT Filed: |
April 28, 2008 |
PCT NO: |
PCT/GB2008/050305 |
371 Date: |
October 19, 2010 |
Current U.S.
Class: |
324/207.15 |
Current CPC
Class: |
G01D 5/208 20130101 |
Class at
Publication: |
324/207.15 |
International
Class: |
G01B 7/14 20060101
G01B007/14 |
Foreign Application Data
Date |
Code |
Application Number |
May 10, 2007 |
GB |
0708981.6 |
Jul 18, 2007 |
GB |
0713942.1 |
Claims
1. A transducer for use in a position sensor, the transducer
comprising: a magnetic field concentrating member; a first coil
operable, in use, to couple with a first portion of the magnetic
field concentrating member positioned adjacent the first coil; a
second coil operable, in use, to couple with a second portion of
the magnetic field concentrating member positioned adjacent the
second coil, the second portion being spaced along the magnetic
field concentrating member from the first portion; and a resonator
having a coil wound around a portion of said magnetic field
concentrating member which is located between said first and second
portions; wherein, during use, the electromagnetic coupling between
the resonator and at least one of the first and second coils varies
as a function of the relative position between the elongate field
concentrating member and that coil.
2. A transducer according to claim 1, wherein the electromagnetic
coupling between said resonator and the first coil varies as a
function of the relative position between the resonator and the
first coil.
3. A transducer according to claim 1, wherein the electromagnetic
coupling between said resonator and the second coil varies as a
function of the relative position between the resonator and the
second coil.
4. A transducer according to claim 1, comprising a third coil
operable, in use, to couple with a third portion of the elongate
magnetic field concentrating member positioned adjacent the third
coil, the first portion being positioned between said second and
third portions.
5. A transducer according to claim 4, wherein said second and third
coils are connected in series.
6. A transducer according to claim 5, wherein said second and third
coils are connected in series so that signals induced in the second
coil by a background magnetic field oppose the signals induced in
the third coil by the same background magnetic field.
7. A transducer according to claim 5, wherein, during use, an
electromagnetic field generated by said resonator couples with a
first polarity with said second coil and couples with a second,
opposite, polarity with said third coil.
8. A transducer according to claim 4, wherein said resonator coil
is mounted asymmetrically on said elongate field concentrating
member.
9. A transducer according to claim 1, for use in a rotary position
sensor and wherein the field concentrating member is rotatable
about an axis which passes through or near said first portion.
10. A transducer according to claim 9, comprising a third coil
operable, in use, to couple with a third portion of the elongate
magnetic field concentrating member positioned adjacent the third
coil, the first portion being positioned between said second and
third portions and wherein said second and third coils are
symmetrically arranged about an axis and wherein said first and
second members are mounted so that said rotation axis is
substantially coaxial with said axis of symmetry.
11. A transducer according to claim 1, wherein said first portion
is located approximately midway along the length of the field
concentrating member and wherein the second portion is located
approximately at one end of the field concentrating member.
12. A transducer according to claim 1, wherein said first and
second coils are substantially planar and wherein the magnetic
field concentrating member is oriented so that a longitudinal axis
of the magnetic field concentrating member lies in a plane that is
substantially parallel with said planar coils.
13. A transducer according to claim 1, wherein said first and
second coils are formed from conductor tracks on a printed circuit
board or conductive inks on a substrate.
14. A transducer according to claim 1, wherein said first coil is
for exciting the resonator and the second coil is for sensing a
signal generated by the resonator when excited.
15. A transducer according to claims 1, wherein said second coil is
for exciting the resonator and the first coil is for sensing a
signal generated by the resonator when excited.
16. A transducer according to claim 1, wherein coupling between the
resonator and the first and/or second coil varies with position in
a direction that is substantially orthogonal to an axis of the
field concentrating member.
17. A position sensor for sensing the relative position of first
and second relatively moveable members, the position sensor
comprising a transducer claim 1 for use in a position sensor, the
transducer comprising: a magnetic field concentrating member; a
first coil operable, in use, to couple with a first portion of the
magnetic field concentrating member positioned adjacent the first
coil; a second coil operable, in use, to couple with a second
portion of the magnetic field concentrating member positioned
adjacent the second coil the second portion being spaced along the
magnetic field concentrating member from the first portion; and a
resonator having a coil wound around a portion of said magnetic
field concentrating member which is located between said first and
second portions; wherein, during use, the electromagnetic coupling
between the resonator and at least one of the first and second
coils varies as a function of the relative position between the
elongate field concentrating member and that coil; and excitation
and processing circuitry for exciting one of the first and second
coils and for processing the signal obtained in the other one of
the first and second coils to determine the relative position of
said relatively moveable members.
Description
[0001] The present invention relates to a transducer for use in an
inductive position sensor. The invention has particular relevance
to transducers for linear and rotary position sensor systems which
are applicable to machines, robotics, front panel controls and
instrumentation, and other applications where such position
measurements are required.
[0002] Various electromagnetic transducers have been proposed for
use in sensing position. A traditional approach is the use of
optical encoders. However these devices are expensive and prone to
dirt contamination unless housed, at added cost. More recently
Hall-effect based sensors have become available, dedicated to the
task of absolute position sensing. These are exemplified by Austria
Microsystems' AS5030 integrated circuit, which measures the angular
position of a magnet positioned above. However these sensors are
relatively sensitive to DC magnetic fields present in their
operating environment, for example due to their proximity to motors
and/or the presence of magnets. They are also sensitive to
misalignment of the magnet's rotation axis relative to their own
central axis, which can cause errors in their reported position.
Such misalignment arises through tolerances at manufacture. It is
possible to calibrate out the resulting errors, but the resulting
calibration time is costly. It is possible to mechanically trim out
the misalignment, but the trimming step is costly. It is possible
to tighten the tolerances, but this usually results in the need for
more expensive components.
[0003] Inductive sensors such as those described in U.S. Pat. No.
6,522,128 (the contents of which are incorporated herein by
reference) overcome the problem of DC magnetic field sensitivity by
operating with AC fields. Many of these known inductive sensors use
excitation and sensor coils that are inductively coupled, in use,
to a resonator coil. To keep manufacturing costs down, the coils,
including the resonator coil, are manufactured from conductive
tracks mounted on printed circuit boards. This can result in
relatively low Q factor and hence poor signal levels and hence poor
signal to noise for a given drive power.
[0004] The present invention aims to provide an alternative
transducer design that can be used in an electromagnetic position
sensor.
[0005] According to one aspect, the present invention provides a
transducer for use in a position sensor, the transducer comprising:
a magnetic field concentrating member; a first coil operable, in
use, to couple with a first portion of the magnetic field
concentrating member positioned adjacent the first coil; a second
coil operable, in use, to couple with a second portion of the
magnetic field concentrating member positioned adjacent the second
coil, the second portion being spaced along the magnetic field
concentrating member from the first portion; and a resonator having
a coil wound around a portion of said magnetic field concentrating
member which is located between said first and second portions;
wherein, during use, the electromagnetic coupling between the
resonator coil and at least one of the first and second coils
varies as a function of the relative position between the resonator
and that coil. In this way, separate processing electronics can
process the signals obtained from the transducer to determine the
desired position information.
[0006] The electromagnetic coupling between the resonator and both
the first and second coils varies as a function of the relative
position between the resonator and those coils.
[0007] A third coil may be provided for coupling with a third
portion of the elongate magnetic field concentrating member
positioned adjacent the third coil, the first portion being
positioned between said second and third portions. In this case,
the second and third coils can be connected in series, preferably
so that signals induced in the second coil by a background magnetic
field oppose the signals induced in the third coil by the same
background magnetic field.
[0008] The arrangement of the resonator and the coils is preferably
such that, during use, an electromagnetic field generated by the
resonator couples with a first polarity with the second coil and
couples with a second, opposite, polarity with the third coil.
[0009] In one embodiment, the field concentrating member is
rotatable about an axis which passes through or near said first
portion and the resonator coil is mounted asymmetrically on the
field concentrating member. In this case, the second and third
coils can be symmetrically arranged about an axis which is
substantially coaxial with the rotational axis of the field
concentrating member.
[0010] In a preferred embodiment, the first and second coils are
substantially planar and the magnetic field concentrating member is
oriented so that a longitudinal axis of the magnetic field
concentrating member lies in a plane that is substantially parallel
with said planar coils. The planar coils can be formed, for
example, from conductor tracks on a printed circuit board or from
conductive inks on a substrate.
[0011] Another aspect provides a rotational position sensor
comprising a resonant element mounted for relative rotational
motion about an axis of rotation and which is operable to generate
substantially equal and opposite magnetic fields on opposing sides
of said axis of rotation in response to a substantially
rotationally symmetric excitation magnetic field.
[0012] The resonant element may be formed from an inductor and
capacitor or from a mechanical type resonator such as a
magnetostrictive element. If formed from an inductor and capacitor,
the inductor may include a coil wound onto a permeable member, such
as a ferrite rod. In one embodiment, the coil is asymmetrically
positioned on the ferrite rod relative to the rotation axis of the
resonant element.
[0013] If required, the permeable member may be tilted relative to
substantially planar sensor coils to ensure that the field
magnitudes are equalised on opposing sides of the rotational
axis.
[0014] This aspect also provides a system for inductively measuring
the angular position of the above resonant element. The system may
include one or more patterned sensor coils for the detection of
resonator angle. The patterning of the sensor coils may be arranged
to generate a substantially sinusoidally varying amplitude with
angle in response to resonator rotation.
[0015] The system may include a processor for combining coupling
information from multiple sensor coils to deliver a position
indication substantially immune to misalignment of the resonator
and sensor coils.
[0016] The system may include a substantially rotationally
symmetric coil for powering the resonator. Alternatively, the
resonator may be powered from the patterned coils.
[0017] These and various other aspects of the invention will become
apparent from the following detailed description of exemplary
embodiment described with reference to the accompanying drawings in
which:
[0018] FIG. 1 is a schematic illustration of a rotary position
sensor for determining the angular position of a rotor relative to
stator;
[0019] FIG. 2a schematically illustrates a SIN sensor winding used
in the position sensor shown in FIG. 1;
[0020] FIG. 2b schematically illustrates a COS sensor winding and
an excitation winding used in the position sensor shown in FIG.
1;
[0021] FIG. 3 schematically illustrates the magnetic interaction
between a resonator mounted on the rotor and an excitation winding
and sensor winding mounted on the stator;
[0022] FIG. 4 schematically illustrates the way in which the
signals obtained from the SIN and COS sensor winding varies with
the rotation angle of the resonator;
[0023] FIG. 5a schematically illustrates an error in reported
position caused by an offset between a resonator and the excitation
and sensor windings;
[0024] FIG. 5b schematically illustrates how the offset shown in
FIG. 5a is overcome using the resonator design shown in FIG. 1;
[0025] FIGS. 6a to 6d illustrate the conductive tracks formed on
the four layers of the sensor board which form the COS sensor
winding;
[0026] FIGS. 7a to 7d illustrate the conductive tracks formed on
the four layers of the sensor board which form part of the
excitation winding;
[0027] FIGS. 8a to 8d illustrate all of the conductive tracks on
the four layers of the sensor printed circuit board;
[0028] FIG. 9 is a block diagram illustrating the excitation and
processing circuitry used to drive the excitation winding and used
to process the signals obtained from the sensor windings to
determine the position of the rotor;
[0029] FIG. 10 schematically illustrates an alternative design of
sensor winding;
[0030] FIG. 11 schematically illustrates an alternative design of
resonator having first and second orthogonally mounted coils;
[0031] FIG. 12 schematically illustrates an alternative design of
resonator having four coaxial coils;
[0032] FIG. 13 schematically illustrates an alternative design of
resonator having three coaxial coils;
[0033] FIG. 14 schematically illustrates an alternative resonator
system that uses a magnetostrictive resonator; and
[0034] FIG. 15 schematically illustrates a linear embodiment for
sensing the position of a resonator element along a linear
measurement path.
OVERVIEW
[0035] One embodiment of the invention is illustrated in FIG. 1. A
resonator 1 is built from a coil 3 wound around a ferrite rod 4,
with the ends of the coil 3 being connected in parallel with a
capacitor 5 to form an LC resonant circuit. As shown, the resonator
coil 3 is mounted asymmetrically along the length of the ferrite
rod 4. The length of the ferrite rod 4 is approximately 20 mm with
a diameter of about 2 mm and the outer diameter of the resonator
coil 3 is about 3 mm. The use of a relatively long thin field
concentrating member (ferrite rod 4) wound with a coil 3 of
relatively small diameter means that the resonator 1 has a very
high Q-factor. The resonator 1 is mounted for angular motion
(represented by arrow 7) relative to a sensor circuit board 9, such
that the axis of the coil 3 is substantially parallel with the
plane of the circuit board 9. The axis of rotation of the resonator
1 is the centre of the ferrite rod 4, which is nominally aligned
with the centre of the sensor board 9. The resonator 1 may, for
example, be mounted on a rotor of a motor and the sensor board 9
may be mounted on the stator.
[0036] The resonator 1 is powered by an excitation coil 11
integrated onto the sensor board 9. This excitation coil 11
generates an approximately uniform and rotationally symmetric field
concentrated near the centre of the resonator's ferrite rod 4. The
excitation field is concentrated by the ferrite rod 4. The
asymmetric placement of the resonator coil 3 on the ferrite rod 4
means that it couples with the resulting concentrated field. The
coupling factor is largely immune to the rotational angle of the
resonator 1, due to the radially symmetric nature of the excitation
field. The resonator 1 is therefore forced to resonate at all
angular positions Az, and with a phase relationship to the
excitation field which is largely independent of that angle.
[0037] Once powered to resonance by the excitation field, the
resonator 1 generates its own AC magnetic fields in response. This
field passes along the ferrite rod 4, such that the field at the
end of the rod furthest from the coil 3 ("long end") is
approximately equal and opposite to the field at the end closest to
the coil 3 ("short end"). This resonator field couples into sensor
coils 13 located adjacent the ends of the ferrite rod 4. In this
embodiment, the sensor coils 13 are patterned so that the coupling
of this resonator field with the sensor coils 13 varies with the
angular position of the resonator 1 relative to the sensor coils
13.
[0038] This resonator 1 can be used in conjunction with a wide
range of sensor coil 13 geometries. The fact that the fields from
its two ends have opposite polarity mean that there is no
rotational symmetry so that it is possible for sensing electronics
to determine the resonator's angular position unambiguously over
360.degree.. The fact that these fields are approximately equal in
strength and opposite in polarity means that the sensor can be
highly immune to misalignments between the resonator 1 and the
sensor circuit board 9, provided that the sensor coils 13 used have
equal and opposite sensitivity to the individual misalignment of
field from the long and short ends of the ferrite rod 4.
[0039] One possible sensor coil geometry is illustrated in FIG. 2a,
which shows a one period "SIN" sensor coil 13-1. As shown, the SIN
coil 13-1 is formed from two sets of conductor loops 15-1 and 15-2,
which are connected in series but wound in the opposite sense. FIG.
2b illustrates the excitation coil 11 and a one period "COS" sensor
coil 13-2, which is similar to the SIN sensor coil 13-1 except
rotated by 90.degree.. Each sensor coil 13 has equal and opposite
sensitivity to fields at opposing points equidistant from the
sensor's central axis. This is because of the two sets of loops 15
of each sensor winding 13 are connected in series so that they are
wound in the opposite direction. In the following description, one
set of loops will be referred to as the S+ or C+ set of loops and
the other will be referred to as the S- or C- set of loops and
these are illustrated in FIG. 2. This means that the field from the
long end of the resonator of FIG. 1 will couple in the same sense
into each sensor coil 13 as field from its short end.
[0040] The SIN coil 13-1 is shown separately for clarity, but its
centre actually coincides with the centres of the COS coil 13-2 and
the excitation coil 11. For clarity the number of turns in each
coil has been simplified: the number of turns in each coil is
larger in the actual pattern, and cross connections are less
pronounced.
[0041] Operation
[0042] The operation of the sensor will now be described with
reference to FIG. 3, which shows the resonator 1, a simplified
representation of the excitation coil 11 and two test coils 18-1
and 18-2 positioned either side of the excitation coil 11. The
sensor's excitation coil 11 is located close to the axis of
rotation of the ferrite rod 4. Since the resonator's coil 3 is
offset from this axis of rotation, there is a constant coupling
between the excitation coil 11 and the resonator 1 independent of
resonator angle. When the resonator 1 is resonating, the AC field
generated by the resonator 1 approximates that of a bar magnet.
Field emerges from the short end 16 of the resonator and "flows" to
the long end 18. Since the field is AC, it can be detected by
measuring the EMF developed in coils which couple with the field.
Test coils 20 placed under the short and long ends of the resonator
in the plane of the sensor board 9, would detect equal and opposite
EMFs. In reality there are no such test coils 20, instead the
resonator 1 interacts with the COS and SIN coils 13 illustrated
above.
[0043] When the resonator 1 is aligned at 0.degree. so that its
short end 16 is centered on C+ (the positive set of loops of the
COS coil 13-2 illustrated in FIG. 2b), it couples in the positive
direction with the COS coil 13-2. This situation is illustrated at
the centre of FIG. 4. At the same time, the long end 18 of the
resonator 1 is aligned with the C- set of loops of the COS coil
13-2, whose winding direction is opposite to the C+ set of loops.
This end also couples positively with the COS coil 13-2, since the
field direction at the long end 18 of the ferrite rod 4 is opposite
to that at the short end 16. The net effect is a large positive
coupling between the resonator 1 and the COS coil 13-2 (kCOS). The
coupling between the resonator 1 and SIN coil 13-1 (kSIN) is zero
at 0.degree., because any EMF developed by the resonator in the S+
set of loops is connected in series with an equal and opposite EMF
developed in the S- set of loops.
[0044] The situation is reversed at -180.degree. (which is
identical to +180.degree. in this case). Here the short end 16 of
the resonator 1 and the C+ set of loops of the COS coil 13-2
coincide, and the long end 18 and the C- set of loops of the COS
coil 13-2 coincide. kCOS is therefore now a large negative value.
kSIN remains zero.
[0045] At 90.degree. the short end 16 of the resonator 1 coincides
with the S+ set of loops and the long end 18 with the S- set of
loops. kSIN is therefore a large positive value, and KCOS is
zero.
[0046] The graph in FIG. 4 illustrates the full relationship
between the two sensor coil coupling factors and resonator angle.
As shown, kSIN has a sinusoidal form, and kCOS its cosine
counterpart. As those skilled in the art will appreciate, although
the geometry of the sensor coils 13 are designed so that these
coupling factors will vary in a substantially sinusoidal manner, in
practice they will not vary exactly sinusoidally. However, this
will simply introduce a slight error into the measurements.
[0047] Misalignment Immunity
[0048] We will now consider the effect of a misalignment (Mx) in
the x-direction and (My) in the y-direction for the resonator 1
shown in FIG. 1 and for an alternative design of resonator, where a
shorter ferrite rod 4' is used and the resonator is symmetrically
positioned along the length of the ferrite rod 4'. These
misalignments in the x and y directions are illustrated in FIGS. 5a
and 5b by the vector rm. With the resonator design shown in FIG.
5a, the misalignment (rm) between the resonator's axis of rotation
31 and the sensor board axis 33 will result in an error in the
measured angle. However, with the resonator design shown in FIG. 1,
this measurement error will, to a first approximation, average out
due to the interaction between the opposite ends of the resonator 1
with the different sets of loops forming the sensor coils 13.
[0049] The reason for this will now be explained. A processing
circuit (not shown) connected to the sensor coils 13 determines
position from the relative amplitude of the signals (Acos and Asin)
detected in the COS and SIN sensor coils 13 respectively:
Az_estimate=a tan 2(A cos, A sin)
[0050] The signals induced in the sensor coils 13 will be AC
signals at the excitation frequency. The processing circuit will
therefore process these signals to determine their relative
amplitudes so that they can be used in the above 4 quadrant
arctangent calculation.
[0051] If we now consider the detected signals as being obtained by
two components--one for the field due to the long end of the
resonator 1 and one due to the field from the short end of the
resonator 1, then:
Az_estimate=a tan 2(A cos_short+A cos_long, A sin_short+A
sin_long)
[0052] Provided the magnitude of the vector rm is small (ie the
misalignments in the x and y directions are small) enough that
detected signals from the short and long ends of the resonator 1
remain approximately the same, then this can be approximated
by:
Az_estimate=0.5.times.[a tan 2(A cos_short, A sin_short)+a tan 2(A
cos_long, A sin_long)]
[0053] Denoting:
Az_estimate_short=a tan 2(A cos_short, A sin_short)
Az_estimate_long=a tan 2(A cos_long, A sin_long)
[0054] which are the effective angular positions of the resonator's
short and long ends (which the system can not individually detect
since the underlying signals are summed together by virtue of the
series connection of the sets of loops 15 forming the sensing coils
13), then we can then write:
Az_estimate=0.5.times.[Az_estimate_short+Az_estimate_long]
[0055] Thus the system's reported position is the average of the
effective angular position of the resonator's short and long ends.
By symmetry, if misalignments Mx and My cause an angular error of
Az_error_short in Az_estimate_short, then an equal and opposite
angular error Az_error_long=-Az_error_short will be caused in
Az_estimate_long. The net effect on the value of Az_estimate
reported by the system will therefore be zero, yielding a system
that is largely immune to misalignments.
[0056] Sensor Board Design
[0057] In this embodiment, the sensor coils 13 and the excitation
coil 11 are formed from conductor tracks formed on a four layer
printed circuit board 9. FIGS. 6a to 6d illustrate the conductors
on the four layers which, when connected together at the
illustrated via holes, form the above described COS sensor coil
13-2; FIGS. 7a to 7d illustrate some of the conductor tracks that
define the above described excitation winding 11; and FIGS. 8a to
8d illustrate the four layers of conductor tracks (showing all
tracks).
[0058] FIG. 6a illustrates a portion of the COS coil 13-2 described
above, implemented on layer 1 of the 4 layer PCB 9. FIG. 6b
illustrates another small portion of the COS coil 13-2--a small
trace connecting two vias at the top centre of the board 9
implemented on layer 2 of the PCB 9. FIG. 6c illustrates another
small trace connecting two vias at the top centre of the board,
implemented on layer 3 of the PCB 9. FIG. 6d illustrates the
remainder of the COS coil 13-2, implemented on layer 4 of the PCB
9. The COS coil portions on layers 1 and 4 have a large number of
coil turns--6 each in this case, yielding a total of 12 turns. This
contrasts with some prior art approaches, where it is not possible
to fit more than about 3 turns in the same diameter operating to
the same design rules. These design rules include he minimum gap
between conductors on the same layer and the minimum diameter for
the vias.
[0059] The two small traces illustrated in FIGS. 6b and 6c are used
to connect the two halves of the COS coil 13-2, and to ensure that
the winding direction of the left and right hand set of loops 15
are opposite, as required.
[0060] Each main sensing set of loops of the COS coil 13-2
comprises a set of turns on layer 1 (here illustrated in FIG. 6a)
and another set on layer 4 (shown in FIG. 6d). The loops on each of
these layers are almost exact mirror images of each other, with a
horizontal axis of symmetry passing through the centre of the
patterns. This symmetry improves the symmetry of the relationship
between resonator angle and output signal, to improve accuracy.
[0061] The two sets of loops are connected by only a few vias. The
use of such a small number of vias means that a maximum amount of
space can be afforded to conductor traces, and hence a maximum
number of turns. This is especially important in the design
presented here, since it is implemented on four layers. Each via
therefore occupies space not only on the layers it is connecting,
but also on the layers occupied by other traces to which
appropriate clearance is required.
[0062] The SIN coil 13-1 is almost identical to the COS coil 13-2,
only rotated through 90.degree. and implemented mainly on layers 2
and 3. FIGS. 8a to 8d show all four layers of the PCB 9 separately,
and illustrate how the SIN, COS and excitation coils, and their
respective cross connections and vias, fit together without
clashing. Once again it is important that the COS coil 13-2 be
almost identical to the SIN coil 13-1, for symmetry reasons that
yield greatest accuracy.
[0063] As shown, the SIN and COS coils 13 are largely implemented
on different layers of the PCB 9. The problem of different sensing
amplitudes is solved by having the SIN and COS coils 13 each
implemented on multiple layers, such that their mean depth within
the PCB stack-up is nominally identical.
[0064] The excitation coil 11 is concentrated towards the centre of
the board 9. FIG. 7a illustrates a first section of the sensor
board's excitation coil 11, extending from the EA connection at the
right of the board to a via at the top right of the coil's
interior. A second section is illustrated in FIG. 7b, extending
from that via to another above the excitation coil 11. A third
section is illustrated in FIG. 7c, extending from that via to
another at the top left of the coil's interior. A fourth section is
illustrated in FIG. 7d, extending from that via to another to the
left of the excitation coil 11. A further 4 sections (not shown in
FIG. 7) of the excitation coil 11 connect in a similar way, using
the two remaining vias at the centre of the excitation coil 11 and
the via to the bottom of the coil, and culminating in a trace
connecting to the EB connection on the right of the board 9.
[0065] The design presented here yields an excitation coil 11 whose
diameter is maximised without clashing with SIN and COS coils 13,
to further increase efficiency and hence output EMF. This is made
possible by eliminating vias positioned between the sensor coils 13
and the excitation coil 11. Vias used for connecting different
sections of the excitation coil 11 are instead positioned within
the turns of and between the SIN and COS coils 13, where there is
plenty of space, or inside the excitation coil 11, where additional
excitation turns have less effect on efficiency.
[0066] FIG. 8 illustrates how the excitation coil 11 is implemented
using the same physical layers as the SIN and COS coils 13 without
clashing. For example, FIG. 8a illustrates layer 1, which comprises
half of the COS coil 13-2 and the second and sixth sections of the
excitation coil 11. These sections of the excitation coil 11 are
designed in such a way that their outer connections occur on layer
1, where they may pass over the SIN coil 13-1 connections on layers
2 and 3 and between the two sets of loops of the COS coil 13-2 on
layer 1. In order to connect in this special way, the excitation
coil 11 is a two-start spiral, with each arm having 5.25 turns.
[0067] The excitation coil 11 can be seen to have substantial
symmetry. In particular, its field patterns are highly symmetric
under rotations of 90.degree. and 180.degree., which means that
coupling between the excitation coil and SIN and COS coils 13 is
minimised. This reduction in stray coupling yields high accuracy,
especially when the sensor is operated with excitation current
applied continuously.
[0068] FIG. 9 schematically illustrates the excitation and
processing circuitry 51 used in this embodiment to determine the
position of the resonator relative to the sensor windings 13. As
shown, the circuitry 51 includes excitation drive circuitry 53 for
generating an excitation signal for application to the excitation
winding 11 and analogue front end processing circuitry 55 for
processing the signals induced in the sensor windings 13. The
circuitry 51 also includes a microprocessor 57 for controlling the
excitation drive circuitry and for processing the signals received
from the analogue front end circuitry to determine the desired
position information, which it then outputs, for example to a host
device. As those skilled in the art will appreciate, various
different ways of driving the excitation winding 11 and of
processing the signals obtained from the sensor windings 13 are
described in the literature and may be used with the sensing
transducer described above. A further description of this circuitry
51 will therefore be omitted.
[0069] Modifications and Alternatives
[0070] The one period sensor windings 13 illustrated in FIG. 2 have
angular accuracy and resolution limited to a certain fraction of
its repeat angle, 360.degree.. For greater accuracy and resolution
multi-period sensor coils can be used instead of or in addition to
the sensor coils 13 discussed above. In order to couple with the
resonator 1, the opposing fields from the short and long ends of
the resonator 1 should couple with loops wound in opposite senses.
This can be achieved by using sensor coils 13 having an odd number
of periods in 360.degree., for example the 3-period windings shown
in FIG. 3a of U.S. Pat. No. 6,534,970, the content of which is
incorporated herein by reference. Only the one of the windings is
schematically shown. A similar winding, except rotated by
30.degree. (one quarter of one third of 360.degree.) could also be
provided.
[0071] As noted above such a 3-period sensor does not detect
angular position unambiguously over 360.degree.. To do this, the
sensor coils of FIG. 2 can be combined onto the same sensor board.
The electronic circuitry 51 connected to the sensor coils would
then combine the unambiguous yet relatively inaccurate data from
the one period sensor coils 13 and the accurate yet ambiguous data
from the three period sensor coils to yield data which is accurate
and unambiguous over 360.degree..
[0072] This approach can be deployed with other sensor coil
combinations. For example the data from three period and seven
period sensor coils can be combined in the same way to yield data
which is accurate and unambiguous over 360.degree..
[0073] The description above is for sensors having two phase
(SIN,COS) sinusoidal sensors. The use of a two phase system is not
necessary; the invention works with three or more phases.
[0074] The description above is for sensor coils that are balanced,
ie where each coil comprises individual coil segments which are
wound in opposite senses. This is not a necessary requirement, as
illustrated by the sensor in FIG. 10. Here there are three sensor
loops a, b and c, each rotational copies of a simple wound sensor
coil. This sensor can be used to detect the position of the
resonator of FIG. 1. In such an embodiment, an electronic processor
would detect the amplitude of signals induced by the resonator 1 in
each loop, respectively Aa, Ab and Ac, and use these to determine
the resonator position using an appropriate interpolation
algorithm. For example the following may be used:
Az_estimate=a tan 2[Aa-(Aa+Ab+Ac)/3, (Ab-Ac)/sqrt(3)]
[0075] Similarly, sensors having a different number of arrayed
coils may be used, in conjunction with interpolation routines
appropriate to the sensor and resonator geometry.
[0076] We noted above that a sensor coil patterned to generate an
odd number of sinusoidal repeats over a circle has equal and
opposite sensitivities to fields generated at points on opposing
sides of its axis, and therefore couples with the resonator of FIG.
1. This approach will not work for a sensor coil pattern having an
even number of repeats, since these have equal sensitivity at the
same phase for points diametrically opposing. Fields from the short
and long ends of the resonator shown in FIG. 1 therefore cancel out
when it is placed across the sensor's diameter. Instead, the
resonator 1 may be placed slightly off the diameter, such that its
short and long ends always coincide with areas where the sensor has
equal and opposite field sensitivity.
[0077] We described above an excitation coil 11 that is wound
within the inner portions of sensor coils 13. The excitation coil
may instead overlap sensor coils as illustrated in U.S. Pat. No.
6,522,128, or it may be wound around the sensor's outer portion, or
a combination of such positions. The excitation coil 11 may be
wound in different senses. For example it may be implemented in two
parts, one wound clockwise within the sensor coils and one
anticlockwise outside. By appropriately selecting the number of
turns in each portion, the fields from the two portions may cancel
at some distance and beyond, to minimise emissions that may
otherwise cause interference.
[0078] We described sensor boards 9 built from conductors printed
on circuit boards (PCBs) above. They may instead be built from
appropriately patterned windings of wire. They may also optionally
be wound onto formers, which may optionally be magnetically
permeable formers to concentrate the field to improve signal levels
and/or accuracy.
[0079] We described a resonator 1 above, having a single offset
winding 3 (which may optionally be tilted), to create equal and
opposite fields at its two ends in response to an excitation field
that is substantially uniform under rotation. This can also be
achieved with the resonator of FIG. 11. Here there are two coils
3-1 and 3-2 connected in series, one around the ferrite rod 4 for
creating equal and opposite fields at the ferrite rod's ends and
one wound around the sense angle axis 59 for relatively uniform
coupling with the sensor board's excitation coil 11. The series
connected coils 3 are placed in parallel with the capacitor 5 as
before, to create a resonant circuit.
[0080] The ferrite rod 4 of FIG. 1 is wound with a single coil 3.
It may instead be built using multiple connected coils 3 to achieve
its function, for example as illustrated in FIG. 12. In this case
the winding directions (represented by the direction of the arrows
61-1 and 61-2) of coils 3-1 and 3-2 towards the centre of the
ferrite rod 4 are opposed to couple with excitation field 63, while
those at opposite ends (coils 3-3 and 3-4) are in the same
direction (represented by arrows 61-3 and 61-4) to create equal and
opposite fields at those ends for coupling with sensor coils as
before.
[0081] The resonators described above were designed to couple with
an excitation field applied near their centres, and to generate
equal and opposite fields at their ends in order to couple with
sensor coils. These positions can be modified. For example the
resonator of FIG. 13 is excited at its ends and still creates equal
and opposite fields at points on opposite and equidistant sides of
the sense axis 63 in response. In particular, the resonator shown
in FIG. 13 includes 3 series connected portions 3-1 to 3-1, with
outer portions 3-1 and 3-3 being wound in opposite sense for
coupling with an excitation coil 11' mounted outside of sensor
coils (not shown). The centre portion 3-2 of the resonator coil 3
is wound in one direction so that this portion of the coil 3 will
generate a resonator field 65 that has equal and opposite fields at
different points either side of the sense axis 63.
[0082] We described magnetically coupled resonators above which
were implemented with a wound coil in parallel with a capacitor,
where energy is exchanged between magnetic (current in a coil) and
electrostatic (electric field across capacitor plates) forms. The
magnetically coupled resonance may be achieved in different ways.
For example, a strip 67 of appropriately biased magnetostrictive
material may be mounted to permit mechanical oscillation 68 as
illustrated in FIG. 14. An excitation field is concentrated inside
the magnetostrictive element 67. A bias element 69 is also provided
which interacts with the excitation field to create mechanical
strains 68 near the centre of the magnetostrictive element 67. The
strains shown are equal in size and the same polarity due to the
reversal in the bias field near the centre. They are relatively
uniform across rotation of the magnetostrictive element 67 about
the sense angle axis represented by the dashed line. The excitation
field's frequency is chosen to coincide with an appropriate
mechanical resonant mode of the magnetostrictive element 67. The
mode shown to the right of FIG. 14 is the fundamental. In
combination with bias fields near the ends of the magnetostrictive
elements 67, this resonance induces fields at these points which
are equal and opposite, as for the resonators of FIGS. 1, 11 and
12.
[0083] The resonators described above may be used in conjunction
with shielding material on the side opposite to the sensor, to
minimise field distortion and interference from objects behind.
This shielding may be built from magnetically permeable material
and/or from conductive material.
[0084] The systems described above sensed position by powering a
resonator with an approximately uniform field and detecting the
response in two or more patterned sensor coils. However what is
fundamental is the measurement of the relative coupling factors
from the resonator to the patterned sensor coils. These coupling
factors may also be derived from a measurement in the "reverse"
direction. In this case current is passed through the patterned
coils (eg coils 13 shown in FIG. 2) to power the resonator 1 and
detected by a coil with relatively uniform coupling (eg coil 11
shown in FIG. 2). Coupling factors may be established by amplitude
measurements taken by powering each patterned coil in turn, or by
modulating the excitation amplitude and/or phase driven into each
patterned coil such that the combined signal out of the uniform
coil may be decoded to separate out the individual coupling
components. Patterned coils may also be used for both energising
the resonator and for sensing the resonator signal, in a similar
manner to the way taught in U.S. Pat. No. 6,489,899, the content of
which is incorporated herein by reference.
[0085] In the previous embodiments, the resonator has been mounted
for rotation relative to the sensor and excitation windings. The
invention is also applicable to linear embodiments, such as the one
illustrated in FIG. 15. In particular, FIG. 15 shows a resonator 1'
which is mounted for movement along the illustrated x-axis, as
illustrated by the arrow 71. As shown, in this embodiment, the
resonator coil 3 is symmetrically mounted on the ferrite rod 4. the
resonator 1' is mounted so that the lower end 73 moves adjacent the
excitation winding 11' and the upper end 75 moves adjacent the SIN
and COS sensor windings 13'-1 and 13'-2. As shown, in this
embodiment, the resonator 1' moves such that its axis 77 is
substantially perpendicular to the measurement path.
[0086] The operation of this embodiment is similar to the previous
embodiments. In particular, when excitation current is applied to
the excitation winding 11', excitation magnetic field couples into
the lower end 73 of the ferrite core 4, which in turn causes the
resonator 1' to resonate. The magnetic field generated by the
resonator 1 will emerge from the lower end 73 of the resonator and
"flows" to the upper end 18, thereby coupling with the SIN and COS
sensor windings 13'. FIG. 14 also illustrates the way in which the
amplitudes of the signals induced in the sensor windings 13' vary
with the position of the resonator 1' along the x-axis. As before,
the amplitudes vary approximately sinusoidally with position.
[0087] In an alternative linear embodiment, a second set of sensor
windings may be arranged below the excitation winding 11'. In this
case, the resonator coil 3 would remain in its current position,
but the ferrite rod 4 would be extended so that the lower end 73
was positioned over the second set of sensor windings. The signals
from the two sets of sensor windings can then be combined and
processed to determine the desired position information. For
example, if the additional set of sensor windings are exact copies
of the windings illustrated in FIG. 15, then the signals from the
two SIN windings can be subtracted and the signals from the two COS
windings can be subtracted to leave signals that vary in the
desired manner. The subtraction can be performed in the processing
electronics or by connecting the two SIN windings in series and the
two COS windings in series. As the resonator field will couple into
the lower set of sensor windings with opposite polarity to that of
the upper set of windings, this subtraction will result in the
addition of the desired positionally varying signals, whilst
removing any common interference. Additionally, this arrangement
will provide the sensor with some immunity to variations of
reported position due to slight rotation of the resonator 1' about
an axis orthogonal to the page. This is because any such rotation
will result in equal and opposite position shifts in the sensor
signals that are combined and therefore, these shifts will
approximately cancel each other out. In such an embodiment, of
course, the second set of sensor windings do not need to be
identical to the first set of sensor windings. The second set of
windings may, for example, have different periodicities so that the
two sets of signals can be combined to overcome the ambiguity
problem associated with windings that repeat.
* * * * *